1. Field of the Invention
The present invention relates to a fuel injection control device for a direct fuel injection engine. More specifically, the present invention relates to a fuel injection control device for performing a stable stratified combustion when engine temperature is relatively low and/or when a fuel pressure is relatively low.
2. Background Information
In conventional direct fuel injection engines, if fuel is injected during an intake stroke when the temperature of the engine is relatively low, e.g., cold starting condition, or when the fuel pressure is relatively low, then the fuel injected into the combustion chamber forms a homogeneous air-fuel mixture with some of the fuel stick to the cylinder wall. As a result, the amount of fuel that does not contribute to combustion increases. Therefore, a total fuel injection amount has to be greatly increased, which causes a large amount of HC to be produced. In order to solve this problem, Japanese Laid-Open Patent Publication No. 2001-271688 discloses a method of starting a direct fuel injection engine for achieving stratified combustion during cold starting by injecting the fuel during the compression stroke. More specifically, the method described in the above mentioned reference discloses a piston top surface provided with a cavity having a substantially cylindrical peripheral wall surface, a bottom wall surface smoothly connected to the peripheral wall surface, and a substantially cone-shaped bulge portion smoothly connected with the bottom wall surface. Moreover, a spark plug is positioned in substantially directly above the bulge portion. In the above mentioned reference, a fuel injection valve is positioned configured to directly inject a fuel stream into a combustion chamber with a substantially hollow circular cone shape from an upper part of the combustion chamber. Since the fuel is injected during the compression stroke in the direct fuel injection engine disclosed in the above mentioned reference, the amount of fuel that sticks to the cylinder wall and the amount of HC produced are reduced.
Moreover, in order to steadily ignite and combust the agglomerate air-fuel mixture in the direct fuel injection engine in the above mentioned reference, it is important for an agglomerate air-fuel mixture having the appropriate size and air-fuel ratio for the engine operating conditions (rotational speed and load) to be formed above the cavity. From the standpoint of reducing exhaust emissions, it is also desirable for the air-fuel ratio to be uniform throughout the agglomerate air-fuel mixture. If rich portions exist within the air-fuel ratio distribution of the agglomerate air-fuel mixture, unburned hydrocarbons (HC) and carbon monoxide (CO) will increase and localized high combustion temperatures will cause NOx to be produced. On the other hand, if there are lean portions within the air-fuel ratio distribution, the lean portions will quench the flame and cause unburned HC and CO to increase.
Consequently, in the method described in the above reference, the fuel is injected from the fuel injection valve during the compression stroke to form a substantially hollow circular cone shaped fuel stream that collides against the peripheral wall surface of the cavity. As the fuel that has struck against the peripheral wall surface of the cavity vaporizes, the fuel gathers toward the bulge portion of the cavity and forms an agglomerate air-fuel mixture in the space above the cavity.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved fuel injection control device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.